Recently, digital still cameras (hereinafter referred to simply as digital cameras) that are capable of inputting image information such as photographed scenery and portraits into a personal computer have rapidly become popular along with the popularity of personal computers in homes. Portable telephones with cameras incorporating small image pickup modules have also rapidly become popular. Additionally, devices that include image pickup modules in small information terminal equipment such as PDAs have also become popular.
Image pickup elements such as CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductors) elements have been used in devices with image pickup modules as described above. For these image pickup elements, great progress has been made recently in both miniaturizing the elements and in increasing the number of image pixels. Compact construction of the main body of the image pickup equipment, including lenses used for forming images, and high resolution in the imaging optics has also been demanded. For example, a portable telephone with a camera providing megapixel (1 million or more pixels) imaging has been practically used, resulting in requirements for increased performance.
An optical zoom mode and an electronic zoom mode are available for realizing the zoom function in image pickup equipment using image pickup elements. In the optical zoom mode, the image size is varied optically by using a zoom lens as the image pickup lens. In the electronic zoom mode, the size of an image is electronically changed by electronic processing of electrical signals produced from an image. In general, the optical zoom mode can provide higher resolving properties than the electronic zoom mode. Therefore, when zooming needs to be performed with high resolution, the optical zoom mode is preferable.
For example, Japanese Laid-Open Patent Application 2003-270533 discloses zoom lenses that are smaller than previous zoom lenses used in digital cameras. The zoom lenses disclosed in this publication include five or six lens elements included in two lens groups.
In general, fixed focus lenses have been used in small information terminal equipment such as portable telephones with cameras based on requirements of miniaturization and low cost, but increased functionality of such equipment has demanded a zoom function. Therefore, the zoom function has been realized recently by adopting an electronic zoom mode in portable telephones that include cameras with fixed focus lenses. However, with this electronic zoom mode, it is difficult to make full use of the large number of image pixels available in image pickup elements now available. The greater the enlargement in the electronic zoom mode, the more the resolution deteriorates.
Accordingly, it is considered desirable to utilize an optical zoom mode by using a zoom lens in a portable telephone that includes a camera. However, it is not practical to use a high performance zoom lens developed for a conventional digital camera because of its large size and high cost. The zoom lenses disclosed in Japanese Laid-Open Patent Application 2003-270533 above achieve miniaturization with a small number of lens elements for use in digital cameras, but in the use of small information terminal equipment, further miniaturization is preferable. On the other hand, a low cost compact zoom lens constructed with about three lens elements has been developed, but it is not designed for operation with image pickup elements currently available that have a very large number of image pixels.